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# "Reverse" perspective and viewing transformation?

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Hi! I''d like to convert screen coordinates to world coordinates. Is it possible with just the screen position (x, y) and the corresponding depth value? I''m using OpenGL and I wrote functions for inverting matrices already. I''d like to use this to, for example, select a specific position on the heightmap with the mouse. Thanks! Drag0n "Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the universe trying to build bigger and better idiots. So far, the universe is winning..." -- Rich Cook

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Anybody?

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the universe trying to build bigger and better idiots. So far, the universe is winning..." -- Rich Cook

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look for gluunroject

it convertes your screen coordinate to worldcoordinate

you need to pass the view and projection matrix ...
read it up on you own

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Ok...that would be gluUnProject(), right? But I was actually looking for how to do it myself!

Thanks anyways...
Drag0n

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gluunproject is tested and very reliable and available with opengl header files

you can reinvent the wheel of course but that won t make it better

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I think what you need to do here is to take the inverse of your projection matrix (shouldnt be too hard to find a tutorial on calculating inverse matrices) and apply that.
If you are not using a projection matrix you''ll just have to undo the arithmetic you do when projecting...

for example
screen_x = (3d_x * x_scale / 3d_z) + center_x;
becomes
3d_x = (screen_x - center_x) * 3d_z / x_scale;

Marijn

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Ok...maybe I didn''t read the Red Book close enough. But is the projection transformation the last operation OpenGL performs vertices? I didn''t think so...

Drag0n

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OpenGL basically applies the following transformations to vertex coordinates (from the OpenGL 1.4 specs, section 2.10):

Po is your initial vertex in object space.

Modelview matrix:
Pe = Modelview * Po

Projection matrix:
Pc = Projection * Pe

Perspective division:
Pd.x = Pc.x / Pc.w
Pd.y = Pc.y / Pc.w
Pd.z = Pc.z / Pc.w

Final viewport transform:
Pw.x = (px/2)*Pd.x + ox
Pw.y = (py/2)*Pd.y + oy
Pw.z = ((f-n)/2)*Pd.z + (n+f)/2

px,py are the viewport''s width and height, ox,oy are it''s center. n,f is the depth range set by glDepthRange (default: n=0, f=1).

You have to perform all those steps backwards.

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